Polyolefin-based elastomers having good fluid and heat resistance are a target of great commercial interest since such materials have potential to serve as cost-advantaged competitors to commercial oil-resistant elastomers, such as nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), and thermoplastic vulcanizates (TPVs), such as Santoprene. In contrast, ethylene-propylene-diene monomer (EPDM) copolymers and other polyolefin-based elastomers do not have sufficient oil resistance to compete in the specialty elastomer market.
The incorporation of functional groups into polyolefins—particularly nitrile groups—is predicted to substantially enhance their fluid resistance. However, the controlled synthesis of nitrile-containing polyolefins through the direct copolymerization (metallocene/Ziegler-Natta, late transition metal, or free-radical) of olefins with acrylonitrile or nitrile-functionalized α-olefins is not currently feasible on a level providing useful catalyst productivities, controlled polymer microstructures, and desirable base polymer properties (see, for example, Padwa, A. Prog. Polym. Sci. 1989, 14, 811-833 and references therein; Mudalige, D. C.; Rempel, G. L. J. Macromol. Sci.—Pure Appl. Chem. 1997, A34, 361-368; Shin, S.-Y. A. et al. Polym. Mat. Sci. Eng. 2004, 91, 100-101; Kochi, T. et al. J. Am. Chem. Soc. 2007, 129, 8948-8949; Marques, M. M. et al. Polym. Int. 2001, 50, 579-587; and U.S. Pat. No. 4,698,403. This is due both to orbital mismatch energies between the monomers and/or the tendency of the nitrile group to poison metal catalyst centers through coordination or side reactions. Typically, olefin/acrylonitrile or olefin/nitrile-functionalized α-olefin copolymers are limited to materials containing small amounts of nitrile comonomer (<6 mol %), alternating copolymers with base properties unlike those of polyolefins, materials having low molecular weights (number average molecular weight, Mn, <20,000), systems that require protection/deprotection steps of the nitrile functionality to mitigate catalyst deactivation, systems that place the polar comonomer groups predominantly at chain ends, or systems that provide very low catalyst productivities.
Polyolefins, particularly polyethylene, are commonly modified by the incorporation of C3 or higher alpha-olefin monomers such as 1-butene, 1-hexene, 1-octene, or 1-decene. These comonomers introduce alkyl branch points along the otherwise linear (and/or stereoregular) polymer main chain, reducing crystallinity and improving properties such as toughness and flexibility through the influence of the alkyl side chain groups residing in the amorphous region. The ethylene/α-olefin copolymers known as Linear Low Density Polyethylenes (LLDPEs) are particularly attractive examples of such materials. When sufficient amounts of α-olefin comonomer are introduced to polyolefins, the level of crystallinity of the resultant copolymers may become so low as to enable the production of useful elastomer or plastomer materials. For such reasons, copolymers of ethylene and nitrile-functionalized α-olefin comonomers, especially ω-nitrile-functionalized α-olefin comonomers (which most closely replicate the structures of linear α-olefin comonomers), are of particular desirability. In these materials, the fluid-resistant functional group and the flexible alkyl chain are introduced simultaneously in one desirable structural unit.
Tandem ring opening metathesis polymerization (ROMP)/hydrogenation techniques have been widely used as an alternate route into functional polyethylene and polyalkenamer structures. To date, these techniques have almost exclusively focused on the synthesis of polymers having functional substituents attached directly to the main chain (constitutionally equivalent to polyolefins produced by the copolymerization of ethylene with a second olefinic monomer having a polar functionality attached directly to one of the polymerizing olefinic carbons). Moreover, these techniques have produced mixed results, with some lower-functionality-tolerant catalysts failing to catalyze polymerization in the presence of the nitrile group. Thus the ROMP of (Z)-cyclooct-4-enecarbonitrile, a functional cyclooolefin monomer that would produce a polymer structure analogous to polyethylene-co-acrylonitrile after tandem ROMP/hydrogenation, was observed to fail when the metathesis catalyst (PCy3)2Cl2Ru═CHCH═CPh2 was used for polymerization (Hillmyer et al. Macromolecules 1995, 28, 6311-6316), but was successfully polymerized with other Ru-based catalysts (PCT International Patent Application WO03/062253A1). The ROMP of nitrile-bearing bicyclo[2.2.1]hept-5-ene-based monomers having one or more methylene spacers between the monomer ring structure and the nitrile functionality has been performed using Ru- and W-based catalysts (for example, see U.S. Pat. Nos. 3,991,139, 3,856,758, and 4,105,608 and WO03/062253A1), in addition to the more common ROMP of the analogous compound with no methylene spacer (5-norbornene-2-carbonitrile). The polymeric ROMP products obtained from the bicyclo[2.2.1]hept-5-ene-based (norbornene-based) monomers, unlike those obtained from monocyclic olefin monomers such as functionalized cyclooctenes, contain intact cyclopentene rings in their backbones.
It has now been found that ROMP of monocyclic olefins having distally pendant polar moieties, especially ω-nitrile-functionalized alkyl substituents, can be used to prepare functionalized polyalkenamers with structures analogous to terpolymers of ethylene, 1,4-enchained butadiene, and an ω-nitrile-functionalized α-olefin. Hydrogenation of these functionalized polyalkenamers produces materials equivalent to copolymers of ethylene and ω-nitrile-functionalized α-olefins. These materials exhibit improved oil swell resistance as compared to unfunctionalized EPDM copolymers, plus superior thermal stability and low-temperature properties (lower glass transition temperature, Tg) as compared to commercial nitrile rubbers. The materials therefore have potential utility in, for example, the production of gaskets and hoses for automobile applications.
The tandem sequential ROMP/hydrogenation of bi- or multicyclic olefin monomers bearing nitrile substituents directly attached to the ring structure is known (for example, see Jpn. Kokai Tokkyo Koho 63317520A and 60049051A). Unlike the ROMP/hydrogenation of nitrile-substituted monocyclic olefins, these techniques produce materials with intact cycloalkane rings in the polymer backbones, rather than materials analogous to the desired ethylene/nitrile-substituted-α-olefin copolymers. Such materials also bear nitrile substituents that are not separated from the atoms in the polymer main chain by desirable, flexible alkyl groups. For example, Yoshida, Y. et al. J. Appl. Polym. Sci. 1997, 66, 367-375 and references therein and U.S. Pat. No. 6,197,894 disclose the ROMP of nitrile-substituted multicyclic monomers such as 8-cyanotetracyclo[4.4.0.13,5.17,10]-3-dodecene, and the subsequent hydrogenation of the resultant nitrile-functionalized polyalkenamers. The Tgs of the hydrogenated materials, as typical of materials containing backbone rings, were high (>135° C.) and suitable for structural rather than elastomeric purposes.
U.S. Patent Application Publication No. 2005/0137369, published Jun. 23, 2005 and the entire contents of which are incorporated herein by reference, discloses a linear functional polymer comprising randomly repeating units A, B and D, wherein A represents —CH2—; B represents —CH(R1), wherein R1 represents a polar functional group; and D represents —C(═O)—, wherein there are at least four A units separating each B unit, each D unit, and each B and D unit, and further wherein when the total number of B units, y, is an integer greater than or equal to 1; and the total number of D units, h, is an integer greater than or equal to 0, then the total number of A units, x, is an integer sufficient that the molar fraction of the B and D units in the polymer is represented by a value j defined by the equation j=(y+h)/(x+y+h)≦0.032. Thus, the polymer product contains low levels of the polar functional group. The polymer is produced by copolymerizing a first polar substituted monomer, such as cyclooct-4-en-1-ol, with a second non-polar unsubstituted monomer, such as cyclooctene, and is a semicrystalline rather than an elastomeric material.